1. Field of the Invention
The present invention relates to a compound having a specific methine-chain structure giving a printed-out image superior in visibility that allows easy differentiation of laser-irradiated and non-irradiated regions, and an image forming material and an image forming method using the same. The invention also relates to an image recording layer of a planographic printing plate precursor using the image forming material, more preferably a planographic printing plate precursor allowing so-called direct plate making in which a plate is directly made by scanning with a laser at a wavelength of 300 to 1,200 nm based on digital signal from computer or the like, a planographic printing plate precursor that can be developed in the printing step directly without being processed in a wet development step, a method of making the same, and a planographic printing method using the same.
2. Description of Related Art
Various compositions and methods for forming an image on a substrate and thus marking the substrate have been used in various industries. As a few examples of such industries, paper making industry, packaging industry, painting industry, medical industry, dental industry, electronic industry, fiber industry, airplane/ship/automobile industries, and visual art industry can be mentioned.
Such image formation or marking is used typically for identification of articles, e.g., for identification of the name or logo of the manufacturer, serial number or lot number, or structural type, or alternatively, for positioning during the production of semiconductor wafers and vehicles such as airplanes, ships, and on-shore vehicles.
Marking is also used in reinforced product, photoresist, solder mask, printing plate, and other photopolymer products. Laser marking is attracting attention recently as a high speed and efficient marking method, and has already been put into practice in some industries. Many laser-marking techniques include irradiating a required region of a substrate with laser radiation to modify or remove the irradiated region, or irradiating a coated substrate with laser radiation and removing the irradiated coating layer to produce contrast between the irradiated region (marked region) and the non-irradiated region (background).
Although laser marking on articles such as semiconductor chip is a rapid and economical means of marking, the latest laser-marking technique of obtaining a desired mark by surface combustion has relevant disadvantages. For example, the mark formed by surface combustion caused by laser is visible only at selected incident angles with respect to the light source. In addition, staining of the mark with oil or other contaminants on the article surface may blur or obfuscate the laser mark. Further, since the laser causes actual combustion on the article surface, in the marking of a bare die, the combustion may damage any structure beneath the surface or damage the internal circuit because of increase in the internal die temperature above its tolerance limit.
Digitalization technology of processing, storing, and outputting image information electronically in and out of computer has been widely spreading recently as an image forming means on the planographic printing plate precursor, and various new image-outputting systems compatible with the digitalization technology have been put into practical use. Computer to plate (CTP) technology of producing a planographic printing plate directly without use of a lith film by making a highly converged radiation ray such as laser radiation carry such digitalized image information and scan-irradiating the planographic printing plate precursor with the ray is attracting attention in the above situation. Accordingly, it is one of important technical issues to obtain a planographic printing plate precursor suitable for such technology.
When making a printing plate from a conventional planographic printing plate precursor, it is necessary to remove undesired portions of the image recording layer through dissolution in a developer or the like after exposure. However there has recently been a demand for elimination or simplification of such an additional wet processing. Under the circumstance, a so-called on-press development method has been proposed as a simplified platemaking method. In the on press development method, an image recording layer allowing removal of undesired portions of the image recording layer of the planographic printing plate precursor in the normal printing step is used, and the undesirable portions of image recording layer are removed on-press after exposure.
However, in general, an on-press developable or no processing (no development) planographic printing plate precursor, which does not accompany pre-printing development, does not have an image on the printing plate when the printing plate is mounted on the printing machine. Therefore, there is a problem in that the plate cannot be identified. In particular, it is quite important in multicolor printing whether resister marks for registration marked on the plate precursor are detectable or not. Accordingly, the use of such an on-press developable or no processing (no development) planographic printing plate precursor requires a means that enables the image to be confirmed after exposure or heating (a printing-out agent).
A printing plate containing, as printing-out agents, a compound that generates acid, base, or radical upon application of light or heat and a compound that changes its color in interaction with the generated acid, base or radical has been proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 11-277927). Moreover, it has been proposed to use the color change of a thermodegradable compound as a printing-out agent for the direct-recording planographic printing plate precursor having a thermosensitive layer (see, for example, JP-A No. 2000-335129). It has further been proposed to use a thermal decomposable colorant having a heat decomposition temperature of 250° C. or lower as a printing-out agent (see, for example, JP-A No. 2003-191657). However, these methods still have a problem in that the visibility in the exposed region is insufficient.